This invention relates to relatively large pneumatic tires which are routinely subjected to severe cutting and chipping ("C & C" for brevity) during use, and more particularly to nylon-reinforced tires. Such large tires are used in off-the-road ("OTR") equipment such as tractors, graders, backhoes, front end loaders, etc., and trucks which serve locations not accessible by conventional concrete or asphalt highways.
Cutting takes place when the tire strikes a sharp object with enough force that the surface is penetrated or cut. Chipping can follow cutting by the effect of tractive, braking, or other forces on rough or sharp surfaces, causing tearing of the rubber compound, usually at 90.degree. to the direction of the cut. Chunking is the tearing away of large chips. Cutting occurs suddenly and is unrelated to tearing; chipping is also relatively sudden but is related to tear strength. Neither cutting nor chipping is related to crack propagation which is a fatigue property.
Natural rubber (NR) has good C & C resistance but generally poor wear resistance, that is, tread wear. Styrene-butadiene-rubber (SBR) also has good C & C resistance but high heat build-up and easy crack propagation. Polybutadiene (PBD) does not propagate crack growth as readily as SBR but has poor C & C resistance. Therefore, PBD is generally avoided in recipes for heavy service OTR tread compounds.
More specifically, this invention is related to large tires having a silica-free tread which has excellent C & C resistance without undue heat build-up and without sacrificing abrasion resistance or tread wear.
Road tests conducted with tires treaded with tread compounds containing carbon blacks of low, normal and high structure showed that, of the properties tested, structure and dispersion of the black exerted greatest effect on abrasion resistance at high wear rates. At rates of wear more severe than 40 miles/mil, the high structure carbon blacks were superior to the normal structure blacks, but at lower severity the reinforcement depended more on particle size that on structure. Since large tires are generally expected to operate at relatively lower severity than 40 miles/mil, the emphasis has always been on particle size. See "Effect of Carbon Black Structure on Abrasion Resistance" by J. R. Haws, C. R. Wilder and W. T. Cooper, Rubber Chemistry and Technology, Vol 42, pg 1495-6 (1969). But good abrasion resistance was not correlatable with C & C resistance. In other words, some tread compounds with good abrasion resistance had unsatisfactory C & C resistance, others with good C & C resistance had unsatisfactory abrasion resistance, and it was difficult to formulate one which had both satisfactory abrasion and C & C resistance.
By "structure" we refer to the degree of fusion of particles into clusters called primary structure aggregates. Once the structure level is produced in the reactor it cannot be increased, but may be decreased by pelletizing or by mixing and processing carbon black compounds. When dibutylphthalate (DBP) is added to carbon black, it covers the surface and is strongly adsorbed into voids and gaps between attached particles. As DBP is added, the black goes from a free-flowing state to that of a paste with a progressive increase in viscosity. An absorptometer senses the changing viscosity and measures the volume of DBP needed to raise viscosity a fixed amount. The more oil required to cause a fixed viscosity increase, the more voids and attached particles and the higher the structure level.
One investigation compared the influence of curative level, total filler loading, adding channel black, silica (AS-100), and an oil, on laboratory C & C resistance. As expected, higher curative levels increased C & C, while higher levels of the other variables decreased them. The replacement of up to 10 phr (parts per 100 parts rubber) of the standard tread black, (PM-100, oil furnace black, USSR), with either channel black or silica produced significant improvement in C & C. See N. L. Sakhnovskii et al, Kauch, Rezina Vol 30(10) pg 30 (1971).
It has become common practice to improve the cutting and chipping resistance of heavy service tires for earth moving and mining equipment, and the like, by using blacks having fine particle sizes characterized by an Iodine No. or Iodine adsorbability (IA) of from 65-120, and high structure characterized by a DBP adsorption (DBPA) in the range from 115-120, and adding 10-25 phr of silica (HS-200) to a conventional tread compound, the more silica used the better the expected C & C resistance of the tread compound. OTR tires made by leading manufacturers of such tires in this country currently use up to 20 phr of silica (see Table I herebelow). However, silica contents in the higher range from about 20 to about 40 phr, increases heat build-up and causes tread separation. Increased silica content also decreases tread wear. To counter a proclivity for tread separation and the decreased tread wear, high structure carbon blacks are used, but structure has only a minor influence on abrasion resistance as severity decreases. But, we use no silica.
It is known that commonly used tread blacks cannot improve both the C & C resistance and wear resistance of rubber, but they will be improved by a particular black having an IA of 130-150 mg/g, a DBPA of 80-105 ml/100 g, and a tinting strength ("tint") of at least 240%, as is disclosed in U.S. Pat. No. 4,154,277 to Sato et al. It is specifically stated there that a black having a DBPA less than 80 ml/100 g lowers the Young's modulus and wear resistance of rubber, while one having a DBPA of more than 105 ml/100 g lowers noticeably the chipping resistance of rubber. Further disclosed therein is that only up to 70% by wt natural rubber (NR) and/or synthetic polyisoprene rubber (IR), the remainder being styrene-butadiene rubber (SBR), may be used, because SBR has poor resistance against heat build up and requires a large amount of processing oil. Yet, we use a high structure black; and, no silica.
The low structure black of Sato et al contributes to its relatively poor cutting resistance, compared to a standard SAF black (see Table 1 of the '277 patent). Superior cutting resistance is of critical importance in an OTR tire since it is difficult to negate crack propagation once the cuts are made. Sato et al's black also contributes to relatively poor tread wear, though this is not apparent in their Table 1. They did not realize that a high structure carbon black could provide excellent C & C resistance without any worse tread wear than obtained with a silica-containing, or low structure black-containing tread.
The low structure, fine particle black of the '277 patent is not known to be generally available commercially, but N293 is available which has a DBPA in the range 80-105 and an IA in the range 135-155, which values correspond to those of the '277 black. When N293 is substituted for the N103 black in the tread of an OTR tire, its performance relative to C & C resistance and tread wear is comparatively unsatisfactory.
More recently, the effect of reinforcing carbon black type, i.e. FEF, HAF, ISAF and SAF, was evaluated in the same NR recipe, inter alia. The C & C results were determined and tabulated in an article titled "A Laboratory Cutting and Chipping Tester for Evaluating Off-The-Road and Heavy-Duty Tire Treads" by J. R. Beatty and J. Miksch, Rubber Chem. Technol. pg 1531-46 Vol 55, No. 5 (1982). In Table VIII of the article are tabulated several blacks, and among those having essentially the same CTAB values (cetyltrimethylammonium bromide adsorption provides a measure of surface area correlatable with particle size) but differing structure, it is evident that there is no correlation between structure and C & C resistance. Among those blacks with a large variation in particle sizes, both C & C resistance and abrasion resistance improve with decreasing particle size (see FIG. 11); however, among those blacks with approximately the same fine particle sizes CTAB 113-116, or 121 there is no correlation of particle size with the level of C & C resistance. It is acknowledged that blacks in the 400 series are too large for satisfactory use in OTR tire treads.